Wash and rinse the jars; put them into a big stockpot; cover the jars with water and bring to a boil; turn off the heat. Let stand in the hot water, covered, until you are ready to fill.

Wash the bands and lids, bring a saucepan of water to boil, add and let sit until you are ready to put them on the jars. (Use new lids each time, bands can be reused.)

For the rhubarb and blueberries –

Add the rhubarb to a tall saucepan with a small amount of water. Heat over medium until the rhubarb is soft and thick. Measure and add to a large saucepan with lemon juice. Mash the blueberries a bit and add in.

To make the calcium water, mix ½ tsp calcium powder (the smaller of the two packets in the pectin box) with ½ cup filtered water in a small canning jar, shake till dissolved. Set aside, you will need 2 tsp of it, the rest can be refrigerated for later use. Add 2 teaspoons of the calcium water to the rhubarb, stir well.

For the honey & pectin –

Measure the honey into a separate bowl and thoroughly mix 2½ tsp pectin (from the larger packet) into it, set aside.

To Cook –

Bring the rhubarb mixture to boil over medium-high, stirring often. Pour the pectin-honey mixture into the boiling jam, slowly and carefully, stirring as you add. Stir vigorously 2 minutes to dissolve the pectin.

Let it return to a boil and remove from the heat. Pectin gels completely when thoroughly cool, so don’t worry if your jam looks loose while still hot.

To preserve –

Empty the water out of your jars, fill to ¼” of the top (a sterilized canning funnel works great). Wipe the rims with a new damp paper towel, removing any spilled jelly, especially on the rim.

Place a lid on top and hand-tighten a band around each jar, place them into a pot of boiling water (such as a canning pot), using a canning rack to lower in. Make sure all jars are upright and that jars are fully submerged, with at least 2″ of water above.

Process for 10 minutes in a boiling water bath. Take out carefully using a jar lifter or tongs. Have a clean kitchen towel on the counter, place each jar on it and let cool for at least 6 hours, overnight is better. Listen for the “popping sound” and keep track of how many times you hear it. Check after cooling that the lid is firm when pressed on, if it pops up and down, it isn’t sealed. If that happens, refrigerate that jar and use within a couple of weeks.

Once cooled, store the jars in a pantry for up to 12 months. Once opened, store in the refrigerator and use up within 3 weeks.